1. Field of the Invention
The present invention generally relates to microfabrication and, in particular, to a system and method for bonding wafers. The present invention is particularly suited for bonding wafers that include circuitry, such as complementary metal-oxide semiconductor (CMOS) circuits.
2. Related Art
As known in the art, there are many situations when it is desirable to use wafer bonding via microfabrication techniques to efficiently manufacture micromachined structures. Furthermore, in many applications, it is also desirable for the bonded wafers to include complementary metal-oxide semiconductor (CMOS) circuitry. However, most conventional wafer bonding processes are not suitable for bonding wafers that include CMOS circuitry and other temperature sensitive components because the relatively high temperatures associated with the bonding process can damage the CMOS circuitry or other temperature sensitive components. In particular, CMOS circuitry can be damaged when exposed to temperatures at or above approximately 500 degrees Celsius. In order to ensure that CMOS circuitry and/or other components are not damaged, it is desirable to bond wafers with temperatures less than or equal to approximately 450 degrees Celsius.
The conventional bonding processes of fusion and oxide diffusive bonding are usually unacceptable for bonding wafers that include CMOS circuitry because these processes typically form bonds with temperatures far above 500 degrees Celsius. Furthermore, any conventional bonding process creating electric fields, such as anodic bonding, is undesirable because the electric fields may break down junctions or gate oxides in the CMOS components. In addition, it is often undesirable to use eutectic bonding in order to bond two wafers together because the existence of liquid phases in these processes can sometimes lead to rapid dissolution of underlayers and, hence, a loss of process control. Furthermore, maintaining precise separation distances between two wafers bonded via eutectic bonding can be difficult since the surfaces of the eutectics typically deform when a temperature close to the eutectic's melting point is reached. Also, organic bonding does not typically provide electrical contacts or good thermal bonds.
Because of the foregoing, conventional bonding processes are not suitable for bonding wafers having CMOS circuitry (or other components sensitive to temperatures above approximately 500 degrees Celsius) unless additional steps are taken to protect the CMOS circuitry and/or other temperature sensitive components. Thus, a previously unaddressed need exists in the industry for providing a system and method of efficiently bonding wafers with CMOS compatible microfabrication techniques.